1. Field of the Invention
The present invention relates to a surface treated steel plate for cans, particularly, to a surface treated steel plate used for the manufacture of a drawn and ironed can, hereinafter referred to as "DI can", i.e., a two-piece can which is manufactured by drawing a disk-shaped steel plate prepared by punching into a cup shape, followed by ironing the barrel portion of the cup-shaped steel plate so as to decrease the thickness thereof.
2. Description of the Related Art
Cans for loading food, which are formed of a tin-plated steel plate, a chromium-plated steel plate, an aluminum sheet, etc., are classified into a three-piece can and a DI can depending on the structure of the can. For manufacturing the three-piece can, a lid portion, a barrel portion and a bottom portion, which are prepared separately, are subjected to various processes including the soldering, welding and fastening processes so as to obtain the desired can. On the other hand, the barrel portion and the bottom portion are formed integrally in a cup shape in the manufacture of the DI can, followed by attaching a lid to the cup-shaped structure. To be more specific, the manufacturing process of the DI can comprises a first stage of drawing a disk-shaped metal sheet prepared by punching into a cup shape, and a second stage of passing the cup-shaped steel plate through a die, i.e., an ironing treatment, such that the barrel portion of the cup-shaped steel plate passes through a small clearance formed between the outer die and the inner punch. Naturally, the thickness of the metal plate in the barrel portion is gradually decreased, and the barrel portion is elongated during the ironing treatment. The treatment including the first and second stages described above is called DI process in the following description.
The DI can prepared by the DI process is smaller in the thickness of the metal plate and lighter in weight than the three-piece can. In addition, seamless between the barrel and the bottom of the DI can prevents a liquid food or the like housed in the can from leaking to the outside. Because of these merits, the demands for the DI can are sharply on the increase and are expected to continue to be on the increase in the future.
Presently, surface treated steel plates such as a tin-plated steel plate and aluminum plates are used as the raw material of a DI can. In particular, the surface treated steel plate is cheaper than the aluminum plate and, thus, the demands for the surface treated steel plate are expected to further increase in the future. However, a very severe processing is required in the step of integrally forming the barrel portion and the bottom portion in the manufacture of the DI can, as described above. Particularly, the plated skin layer forming the surface layer of the surface treated steel plate is subjected to severe conditions. As a result, it is unavoidable for the plated skin layer to be deteriorated, when the tin-plated steel plate or the chromium-plated steel plate used as the conventional can material is subjected to the DI process. In the case of, for example, a tin-plated steel plate, the tin layer covering the surface of the steel plate serves to ensure a high corrosion resistance of the plated steel plate and, at the same time, plays the roll of a lubricant in the ironing process. Thus, the plated tin layer is highly desirable when applied to the outer surface, i.e., the outer surface of the cup-shaped steel plate, of the steel plate used for the DI process.
However, the tin-plated steel plate leaves room for further improvement. Specifically, the plated tin layer on the inner surface of the cup-shaped steel plate is brought into direct contact with the punch in the ironing process, with the result that the plated tin layer is cracked so as to expose the steel plate itself to the outside. The exposure causes deterioration in the secondary corrosion resistance after the coating of the inner surface of the can body, giving rise to the undercutting corrosion. As a result, iron is eluted into the material such as beverage loaded in the can. In this case, the life of the can is shortened. In addition, the taste and flavor of the loaded beverage are impaired.
To overcome the difficulty described above, it was customary in the past to apply a sealing treatment with a phosphoric acid series treating solution or a chromium phosphate series treating solution so as to form a sealing layer covering the exposed surface of the steel plate. However, such a sealing treatment fails to sufficiently improve the corrosion resistance on the inner surface of the can body, making it necessary to apply a coating treatment with an organic film a plurality of times so as to ensure a sufficient corrosion resistance. Needless to say, however, these treatments lead to an increased manufacturing cost of the can.
In some cases, cans manufactured by the DI process are used with no coating applied to the external ring portion at the bottom of the can. Of course, the cans tend to be rusted during the storage or transportation.
In the case of a chromium-plated steel plate, the surface of the steel plate is covered first with a chromium plating layer electrolytically formed by using an aqueous solution and then with a hydrated chromium oxide layer. In this case, the chromium plating layer is hard, with the result that the surface layers are damaged during the DI process, as in the tin-plated steel plate. A measure for overcoming the difficulty is proposed in, for example, Published Unexamined Japanese Patent Application No. 55-82797. It is proposed that a thermosetting coating material is applied to both surfaces of the steel plate so as to allow the coating to perform a lubricating function and to enable the steel plate to exhibit an improved corrosion resistance. However, the improvement in the corrosion resistance is not satisfactory in spite of the presence of the thermosetting coating layer. In addition, the coating layer causes the can body to be adhered to the punch during the ironing process, making it necessary to withdraw the punch with force greater than the mechanical strength of the can body. It follows that a so-called "roll back", i.e., occurrence of warping in the upper peripheral portion of the barrel portion of the can body, tends to take place. Under the circumstances, a chromium-plated steel plate has not yet been put to a practical use for the manufacture of cans.
The present inventor has conducted researches on the DI processing capability and corrosion resistance of a steel plate covered with a single or a plurality of aluminum plating layers for improving the corrosion resistance. It has been found that a mold biting takes place during the DI process, leading to a marked deterioration in the appearance of the manufactured cans.
As described above, the conventional surface treated steel plates are not satisfactory in various aspects. To reiterate, the tin-plated steel plate necessitates a sealing treatment after the DI process. On the other hand, the chromium-plated metal plate requires a coating treatment with a thermosetting coating material. In addition, the metal plate fails to exhibit a sufficiently high corrosion resistance in spite of the presence of the coating layer. Also, an excessive force is required for withdrawing the punch after the DI process, giving rise to the roll back problem. Further, a mold biting takes place in the case of a steel plate covered with an aluminum surface layer, leading to a shortened life of the die.